In stud welding devices predominantly used up to now, a welding head or, for manual welding, a welding gun is employed, with a supporting foot or a spacer being provided at the front end of the welding head or the welding gun, respectively. Thus, after placing the stud welding head or the welding gun upon the surface of the workpiece with the supporting foot, a defined position is obtained, particularly a defined distance between the welding head or the welding gun and the surface of the workpiece. Then, taking this defined position as a starting position, the stud holding device or chuck and the part held therein and to be welded can be moved relative to the surface of the workpiece.
A stud welding device of this kind is described in the document DE 32 15 453 C 1, for example. In this stud welding device, the stud welding gun comprises a drive in the form of an electric motor for the welding axle, with a d.c. motor driving the welding axle (here, the moving part assembly, which comprises all moving parts that cooperate to perform the actual welding movement such as the charging head, the feed cylinder, the feed piston, etc.) and the stud holding device connected therewith in the axial direction. After the welding gun has been placed upon the surface of the workpiece with the supporting foot, the stud holding device is first moved towards the surface of the workpiece until the face of the part held in the stud holding device and to be welded contacts the surface of the workpiece. Then, the current for the pre-current arc or pilot arc is switched on and the welding axle is lifted off from the surface of the workpiece by means of the d.c. motor. Hereby, a pre-current arc is struck. The further course corresponds to the usual process of stud welding with stroke ignition where a pilot arc is struck by placing a stud in good electrical contact with the workpiece, switching on the pre-current and retracting the stud from the workpiece.
Furthermore, from the document EP 0 776 261 B1, a device for welding together at least two parts is known, in which a welding head is oriented opposite to the intended welding area of the workpiece and is then moved towards the workpiece by means of an adjustment drive, e.g. the arm of a robot, until the face of the part to be welded is pressed onto the surface of the workpiece. In the course of this procedure, the welding head is moved towards the workpiece beyond the position in which the part to be welded touches the surface of the workpiece for the first time. For this purpose, the welding axle of the welding head is configured such that it is moved into the welding head when the welding head is moved further towards the workpiece in this way. By providing such an additional path of movement for the welding head, it is possible to make steady contact between the face of the part to be welded and the workpiece within a large range independently of positional tolerances of the workpiece and of tolerances occurring during the movement of the welding head. After the movement of the welding head has been stopped, the welding axle of the welding head can be moved together with the stud holding device and the part to be welded held therein, starting from the starting position thus created, in order to continue the welding operation. This movement is effected by means of the drive for the welding axle. Thus, this device and this method make welding without a supporting foot possible.
Welding without a supporting foot has the advantage, among others, that the welding area, i.e. the area of the surface of the workpiece which has to be accessible for welding, may be markedly smaller compared to welding with a supporting foot. In particular, when welding with a supporting foot, it is hardly possible to weld a part to be welded to a workpiece in relatively narrow grooves of this workpiece.
Particularly in stud welding methods or stud welding devices working without a supporting foot, however, there is a certain danger that if there is no part to be welded in the stud holding device, the stud holding device itself will be welded to the workpiece.
In stud welding devices working with a supporting foot, the presence of a part to be welded in the stud holding device could easily be detected by measuring after placing the welding head upon the workpiece—the distance which the welding axle has to cover when it is moved towards the workpiece until a contact is made. Then, knowing the geometry of the part to be welded and its position in the stud holding device, one can draw conclusions on the presence of a part to be welded, if the distance covered by the welding axle until the part to be welded contacts the workpiece is smaller than a pre-determined distance. This method, however, fails in case of stud welding devices working without a supporting foot.
Accordingly, the object of the invention is to provide a stud welding method and a stud welding device, particularly for stud welding without a supporting foot, in which the presence of a part to be welded in the stud holding device is securely detectable.
The invention achieves this object with the features of patent claims 1 and 10 and 15. Furthermore, the invention provides a program, particularly for upgrading existing stud welding devices, in order to be able to perform the method according to the invention.